Lithographic apparatus and device manufacturing method

ABSTRACT

An immersion lithographic apparatus is disclosed in which a gas knife is shaped and a liquid removal device is positioned to improve removal of liquid from the surface of the substrate.

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 11/436,057 filed May 18, 2006, the entire contentsof which is hereby incorporated by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852) means thatthere is a large body of liquid that must be accelerated during ascanning exposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent application WO99/49504. As illustrated in FIGS. 2 and 3, liquid is supplied by atleast one inlet IN onto the substrate, preferably along the direction ofmovement of the substrate relative to the final element, and is removedby at least one outlet OUT after having passed under the projectionsystem. That is, as the substrate is scanned beneath the element in a −Xdirection, liquid is supplied at the +X side of the element and taken upat the −X side. FIG. 2 shows the arrangement schematically in whichliquid is supplied via inlet IN and is taken up on the other side of theelement by outlet OUT which is connected to a low pressure source. Inthe illustration of FIG. 2 the liquid is supplied along the direction ofmovement of the substrate relative to the final element, though thisdoes not need to be the case. Various orientations and numbers of in-and out-lets positioned around the final element are possible, oneexample is illustrated in FIG. 3 in which four sets of an inlet with anoutlet on either side are provided in a regular pattern around the finalelement.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, each herebyincorporated in their entirety by reference, the idea of a twin or dualstage immersion lithography apparatus is disclosed. Such an apparatus isprovided with two tables for supporting a substrate. Levelingmeasurements are carried out with a table at a first position, withoutimmersion liquid, and exposure is carried out with a table at a secondposition, where immersion liquid is present. Alternatively, theapparatus has only one table.

SUMMARY

It is desirable to improve liquid removal from the surface of asubstrate and/or substrate table.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a projection system configured totransfer a pattern from a patterning device onto a substrate; a liquidsupply system configured to provide liquid to a space between theprojection system and the substrate; a gas knife arranged to at leastpartly surround the space; and a liquid removal device positionedadjacent to and along a fraction of the gas knife, wherein the gas knifeis shaped such that, on movement of the substrate in any direction inthe plane of the substrate and which passes through an optical axis ofthe apparatus, in an arc of at least 36°, liquid on the substrate in aplane which is perpendicular to the plane of the substrate and whichcontains the direction will, by combined effect of the gas knife andfurther movement of the substrate in the direction, be transported alongthe gas knife to the liquid removal device.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a projection system configured totransfer a pattern from a patterning device onto a substrate; a liquidsupply system configured to provide liquid to a space between theprojection system and a substrate; and a gas knife at least partlysurrounding the space to contain liquid left on the substrate by theliquid supply system, wherein the gas knife is shaped, in plan, suchthat at least some portions have a local radius larger than an averageradius of the gas knife, or at least some portions have a local radiussmaller than an average radius of the gas knife, or both.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a projection system configured totransfer a pattern from a patterning device onto a substrate; a liquidsupply system configured to provide liquid to a space between theprojection system and a substrate; and a gas knife sized, shaped andoriented such that, on movement of the substrate under the liquid supplysystem and under the gas knife, liquid left on the substrate followingpassage of the substrate under the liquid supply system is transportedalong the gas knife to a liquid removal area.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a projection system configured totransfer a pattern from a patterning device onto a substrate; a barrierstructure configured to at least partly surround a space between theprojection system and the substrate and to at least partly constrainliquid in the space; and a gas knife positioned spaced apart from thebarrier structure, wherein liquid on the substrate which has escapedfrom the barrier structure has a free surface between the barrierstructure and the gas knife.

According to an aspect of the invention, there is provided an immersionlithographic apparatus, comprising: a projection system configured totransfer a pattern from a patterning device, through a liquid, onto asubstrate; and a liquid removal device configured to remove liquid froma surface of the substrate, the liquid removal device comprising aplurality of extraction tubes connected to an under pressure, with anend, in use, directed towards the substrate and positioned within tentimes the maximum tube end plan dimension of each other.

According to an aspect of the invention, there is provided an immersionlithographic apparatus, comprising: a projection system configured totransfer a pattern from a patterning device, through a liquid, onto asubstrate; a gas knife; and a liquid removal device comprising a tubeconnected to an under pressure, an end of the tube positioned adjacentthe gas knife and in a recess in a plan shape of the gas knife.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising projecting a patterned beam of radiationusing a projection system through a liquid onto a substrate, whereinliquid is contained by a gas knife at least partly surrounding thespace, and wherein the gas knife is shaped, in plan, such that at leastsome portions have a local radius larger than an average radius of thegas knife, or at least some portions have a local radius smaller than anaverage radius of the gas knife, or both.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising projecting a patterned beam of radiationusing a projection system through a liquid, supplied by a liquid supplysystem, onto a substrate and transporting liquid left on the substratefollowing passage of the substrate under the liquid supply system alonga gas knife to a liquid removal area.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising projecting a patterned beam of radiationusing a projection system through a liquid onto a substrate and allowingliquid to escape from a liquid containment device such that it is on thesubstrate with a free surface prior to removal of liquid using a gasknife.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising projecting a patterned beam of radiationusing a projection system through a liquid onto a substrate and removingliquid from a surface of the substrate using a plurality of extractiontubes which are connected to an under pressure, have an end directedtowards the substrate and positioned within ten times of the maximumtube end dimension of each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system in accordance with anembodiment of the present invention;

FIG. 6 illustrates a liquid supply system in accordance with anembodiment of the present invention;

FIG. 7 illustrates a first embodiment of the present invention useablewith the liquid supply system of FIG. 6;

FIG. 8 illustrates a second embodiment useable with the liquid supplysystem according to FIG. 6;

FIG. 9 illustrates a third embodiment useable with the liquid supplysystem according to FIG. 6;

FIG. 10 illustrates a fourth embodiment of the present invention useablewith the liquid supply system of FIG. 6;

FIG. 11 illustrates a first embodiment of a liquid removal deviceaccording to the present invention;

FIG. 12 illustrates a second embodiment of a liquid removal deviceaccording to the present invention;

FIG. 13 illustrates, in cross-section, a third embodiment of a liquidremoval device according to the present invention; and

FIG. 14 illustrates an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or DUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system.”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a barrier structure which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. Such a solution is illustrated in FIG. 5. Thebarrier structure is substantially stationary relative to the projectionsystem in the XY plane though there may be some relative movement in theZ direction (in the direction of the optical axis). In an embodiment, aseal is formed between the barrier structure and the surface of thesubstrate and may be a contactless seal such as a gas seal.

The barrier structure 12 (shown, for example, as element IH in FIG. 1)at least partly contains liquid in the space 11 between a final elementof the projection system PL and the substrate W. A contactless seal 16to the substrate may be formed around the image field of the projectionsystem so that liquid is confined within the space between the substratesurface and the final element of the projection system. The space is atleast partly formed by the barrier structure 12 positioned below andsurrounding the final element of the projection system PL. Liquid isbrought into the space below the projection system and within thebarrier structure 12 by liquid inlet 13 and may be removed by liquidoutlet 13. The barrier structure 12 may extend a little above the finalelement of the projection system and the liquid level rises above thefinal element so that a buffer of liquid is provided. The barrierstructure 12 has an inner periphery that at the upper end, in anembodiment, closely conforms to the shape of the projection system orthe final element thereof and may, e.g., be round. At the bottom, theinner periphery closely conforms to the shape of the image field, e.g.,rectangular though this need not be the case.

The liquid is contained in the space 11 by a gas seal 16 which, duringuse, is formed between the bottom of the barrier structure 12 and thesurface of the substrate W. The gas seal is formed by gas, e.g. air orsynthetic air but, in an embodiment, N₂ or another inert gas, providedunder pressure via inlet 15 to the gap between barrier structure 12 andsubstrate and extracted via outlet 14. The overpressure on the gas inlet15, vacuum level on the outlet 14 and geometry of the gap are arrangedso that there is a high-velocity gas flow inwards that confines theliquid. Those inlets/outlets may be annular grooves which surround thespace 11 and the flow of gas 16 is effective to contain the liquid inthe space 11. Such a system is disclosed in United States patentapplication publication no. US 2004-0207824, hereby incorporated in itsentirety by reference.

FIG. 6 illustrates, in cross-section, an embodiment of a barrierstructure 12 which is part of a liquid supply system LSS. The barrierstructure 12 extends around the periphery of the final element of theprojection system PS such that the barrier structure (which may becalled a seal member) is, for example, substantially annular in overallshape. The projection system PS may not be circular and the inner and/orouter edge of the barrier structure 12 may also not be circular so thatit is not necessary for the barrier structure to be ring shaped and itcould also be other shapes so long as it has a central opening throughwhich the projection beam PB may pass out of the final element of theprojection system PL through liquid contained in the central opening andonto the substrate W.

The function of the barrier structure 12 is to at least partly maintainor confine liquid in the space between the projection system PS and thesubstrate W so that the projection beam PB may pass through the liquid.

The barrier structure 12 comprises a plurality of inlets 50 throughwhich liquid is provided into the space between the final element of theprojection system PS and the substrate W. Liquid may flow over theprotrusion 60 and then be extracted through extractor 70. Thisarrangement can substantially prevent overflowing of the liquid over thetop of the barrier structure 12. The top level of the liquid is simplycontained by the presence of the barrier structure 12 and the level ofliquid in the space is maintained such that the liquid does not overflowover the top of the barrier structure 12.

A seal is provided between the bottom of the barrier structure 12 andthe substrate W. In FIG. 6 the seal is a contactless seal and a deviceto provide the seal is made up of several components. Working radiallyoutwardly from the optical axis of the projection system PS along thebottom 80 of the barrier structure 12 there is provided a single phaseextractor 180 such as the one disclosed in United States patentapplication publication no. US 2006-0038968, incorporated herein in itsentirety by reference. Any type of liquid extractor can be used. In anembodiment, the liquid extractor comprises an inlet which is covered ina porous material which is used to separate liquid from gas to enablesingle-phase liquid extraction. Radially outwardly of the single-phaseextractor 180 is a meniscus pinning feature 500 which in the case of theembodiment illustrated in FIG. 7 is a sharp corner though clearly othermeniscus pinning features may be used. This meniscus pinning feature 500pins a meniscus of liquid 510 at that position. However, a film ofliquid 600 is still likely to remain on the surface of the substrate W.A recess 700 is provided in the bottom surface of the barrier structure12 such that the film of liquid 600 is not constrained and has a freetop surface. Radially outwardly of the recess 700 is a gas knife andliquid extractor assembly 400 which will be described in more detailbelow. An embodiment of the present invention is directed to the gasknife and liquid extractor assembly and can be used with any liquidsupply system, including those illustrated in FIGS. 2-5 and inparticular those types of liquid supply system which provide liquid to alocalized area of the substrate (i.e. those which provide liquid to atop surface area of the substrate W smaller, in plan, than the overalltop surface area of the substrate W and relative to which the substrateW is moved). The gas knife and liquid extractor assembly 400 can formpart of the liquid supply system LSS as illustrated in FIG. 6 or can beseparate from the remainder of the liquid supply system. The singlephase extractor 180 and meniscus pinning feature 500 of the FIG. 6embodiment could be replaced with any other type of (partial) seal.

The gas knife assembly 400 comprises a gas knife 410 which extendsaround the entire periphery of the barrier structure 12 therebysurrounding the space 11. This is not necessarily the case and there maybe areas at which the gas knife 410 is not continuous. Radially inwardlyof the gas knife 410 in the cross-section in FIG. 6 is a liquidextractor 420.

As will be described with reference, in particular, to FIGS. 7-10, theliquid extractor 420 is not positioned peripherally around the entirespace occupied by liquid but is only positioned at discrete locations.Indeed, the liquid extractor 420 is actually comprised of severalindividual discrete liquid extractors positioned at places along the(peripheral) length of the gas knife 410. The locations at which theliquid extractor 420 is positioned can be regarded as stagnation pointswhich are points at which liquid which is moving away from the opticalaxis of the apparatus (along which the projection beam PB propagates) isconcentrated by the shape of the gas knife 410. In an embodiment, theliquid extractor extends along less than 0.05 of the gas knife. Thiswill be described in more detail below.

As can be seen from FIG. 6, the effect of the gas knife is to create abuild-up of liquid 610 just radially inwardly of the gas knife 410. Afast jet of gas is directed by the gas knife 410 in a directionsubstantially perpendicular to the top surface of the substrate W. Thegas knife 410 is designed to move this build-up of liquid, incombination with the moving substrate W, to one of the so calledstagnation points at which a liquid removal device 420 will be ableefficiently to remove the build-up of liquid 610.

The maximum speed at which the substrate W may move under the projectionsystem PS and/or the barrier structure 12 is determined at least in partby the speed at which the build-up of liquid 610 breaks through the gasknife. Thus, this build-up of liquid should be removed before itspressure becomes great enough to force its way past the gas knife 410.This is achieved in an embodiment of the present invention by ensuringthat the build-up of liquid is moved along the gas knife to anextraction point. This allows the liquid extractor 420 to operateefficiently because the build-up of liquid will completely orsubstantially cover its end or inlet 422 such that the extractorextracts exclusively or substantially liquid rather than a mixture ofliquid and gas. In the mode of operation where substantially only liquidis extracted the efficiency of the extractor is increased. This will bedescribed in more detail below with reference to FIGS. 11 a and 11 b.

FIGS. 7-10 illustrate four embodiments in which the shape of the gasknife 410 and position of the individual liquid removal devices 420 arearranged so as to concentrate liquid at the liquid removal devices bycombined effect of the gas knife and movement of the substrate. Ingeneral an embodiment of the invention can be seen as the shaping of thegas knife, in plan, such that, on movement of the substrate W in anydirection which lies in an arc of at least 36° and which direction is inthe plane of the substrate and which direction passes through theoptical axis of the apparatus, liquid on the substrate which isperpendicular to the plane of the substrate and which contains thedirection in which the substrate is moving will, by combined effect ofthe gas knife and further movement of the substrate in the movement ofdirection, be transported along the gas knife to a liquid removaldevice. Each of FIGS. 7-9 show such an arc (in the cases of FIGS. 7 and9 the arc is 45° because there are eight liquid removal devices 420 andin the case of FIG. 8 the arc is 90° because there are only four liquidremoval devices). The directions of movement of the substrate W areillustrated by lines 450 and it will be seen that when liquid initiallyon that line meets the gas knife 410 by continued movement of thesubstrate in the direction 450 liquid will move along the gas knife 410because it cannot get past the gas knife 410 but still has a componentof the velocity of the substrate moving in direction 450 towards theliquid removal device in the respective arc. The direction of movementof the liquid is illustrated by arrows 460. It will clearly beappreciated that in the embodiments of FIGS. 8 and 9 the arc at whichthe liquid is transported to only a single one of the liquid removaldevices is actually less than 90° and less than 45° respectively becauseif the direction 450 is perpendicular to the gas knife 410, the liquidwill not have a component towards either of the liquid removal devicesto its left and right when it impinges on the gas knife 410.

Furthermore, it will be appreciated that the above is simply adefinition of the shape of the gas knife. Liquid which is not positionedon the substrate in a plane perpendicular to the plane of the substrateand which plane contains the direction 450 will also be transportedalong the gas knife 410 when it impinges on it and will be transportedtowards one of the liquid removal devices. An embodiment of theinvention relates to the sizing, shaping and orientation of the gasknife such that, on movement of the substrate under the liquid supplysystem and under the gas knife, liquid left on the substrate followingpassage of the substrate under the liquid supply system LSS will betransported along the gas knife to a liquid removal area. At the liquidremoval area the build-up of liquid 610 will be concentrated such thatliquid extraction at the liquid removal area becomes more efficient.

FIG. 7 shows the gas knife and liquid extractor assembly 400 in plan.The shape of the gas knife 410 is shown in solid lines and the locationof the liquid extractors 420 is also shown. Furthermore, an imaginarycircle 430 has been superimposed (dotted line) which represents anaverage radius of the gas knife 410. As can be seen, the gas knife iscomprised of eight segments each which has a radius of less than theaverage radius of the gas knife so that there are areas of the gas knifewhich are closer to the projection beam PB and areas which are furtheraway than the average radius. The liquid extractors 420 are positionedat those parts of the gas knife which are furthest away just radiallyinwardly of the gas knife as is illustrated in FIG. 6. As will beappreciated, in any position where liquid is moving away from theoptical axis of the projection beam and impinges on the gas knife 410,upon further movement of the liquid in that direction it will be forcedby the gas knife along the gas knife towards a liquid removal device 420as described above. In an embodiment, the one or more of the segmentsmay have a radius that is larger than the average radius. In anembodiment, at least some segments have a local radius of less than 0.9of the average radius or of greater than 1.1 of the average radius. Inan embodiment, the local radius is at least ten times larger than theaverage radius.

It will be noted that all of the liquid collection devices 420 arepositioned further from the projection beam PB than the average radiusof the gas knife.

FIG. 8 illustrates a second embodiment of the gas knife assembly 400 inwhich the gas knife 410 is comprised of four segments which are eachstraight. At each of the meeting points of those four segments there isprovided a liquid removal device 420. In this instance the radius of theindividual straight parts of the gas knife 410 can be considered to beinfinity. The sharp changes in direction can be considered to have aradius of zero. Again, upon movement of liquid away from the opticalaxis of the apparatus, as represented by the projection beam PB, ineither the x (step direction) or y direction (scan direction) the liquidwill be directed by the gas knife to a collection point at the junctionbetween two separate parts of the gas knife 410 where there is a liquidremoval device 420 at which efficient removal of the liquid becomespossible.

In FIG. 9 a third embodiment is depicted in which the gas knife 410 iscomprised of eight portions each of which have a radius which is smallerthan the average radius but which is curved in the opposite directionfrom that shown in FIG. 7. Again the shape of the gas knife 410 is suchthat liquid moving away from the optical axis of the apparatus will bedirected towards the stagnation points at which the liquid removaldevices 420 are situated.

It will be appreciated that the gas knife does not need to be shaped tohave a radius over a significant proportion of its length. For instancethe arrangement whereby a stagnation point is formed at a position alongthe length of the gas knife 410 could also be arranged by using aplurality of segments with the same radius as the average radius. Thiswould be arranged by arranging for the radius of those portions to notshare a central axis with the average radius. However, portions of thegas knife, for example where those individual sections meet, would havea radius which is not equal to the average radius. Thus, it can be seenthat there are many shapes which will provide the desired effect oftransporting the build-up of liquid 610 along the gas knife 410 towardsa stagnation point.

It will furthermore be appreciated that the fewer stagnation pointsprovided around the periphery of the gas knife, the greater the build-upof liquid at the stagnation points and thereby the more efficient theextraction system 420. In an embodiment, there are fewer than 10stagnation points along the periphery of the gas knife 410 (thus the 36°arc). In an embodiment, as illustrated in FIG. 8, the shape of the gasknife 410 is optimized for movements mainly in the y and/or xdirections.

FIG. 10 shows a shape of the gas knife in which the embodiment of FIG. 8is elongated in the y (scan) direction and the stagnation points areprovided on the x and y axes of symmetry of the gas knife 410. As willbe described with respect to FIGS. 11 and 12, it is advantageous thatthe stagnation points at which liquid will be collecting during the scanmotion (which is the motion with the fastest movement) enables evenfaster movement than a totally symmetrically shaped gas knife 410. Thisis because by the elongation of the shape of FIG. 8 the angle normal tothe gas knife 410 has a reduced component in the y direction therebyincreasing the scan speed at which a given quantity of liquid breaksthrough the gas knife.

FIG. 11 a illustrates, in plan, a liquid extractor 420 according to anembodiment of the present invention. Liquid is transported in thedirection of arrows 405 along the gas knife 410 towards the liquidextractor 420. The liquid extractor 420 comprises a plurality of tubeseach with an inlet 422 facing the substrate W (see FIG. 6). Each of thetubes is individually connected to an under pressure. In this way, ifthe build-up 610 of liquid covers the entire inlet 422 of a single tube,that tube will extract 100% liquid which is the most efficient way ofextracting liquid. If the inlet 422 is not covered by liquid, a mixtureof liquid from the build-up 610 and gas from the gas knife and from therecess 700 will be extracted through the tube. This extraction is lessefficient. Thus, by transporting the build-up 610 along the gas knife toan extraction area the likelihood of a single tube having its inlet 422covered by liquid increases. Such a construction can also be used formeniscus pinning with some liquid extraction. The high gas flow rateinto the tubes creates a force on the meniscus to hold it in place.Furthermore, by splitting up the liquid extractors into individualextractor tubes, the chances of a single extractor having its end orinlet 422 completely immersed goes up while the total capacity of theextractor 420 is kept higher than the provision of a single tube but thesame as the provision of a single extractor with the same combined areaof inlets as the plurality of tubes. In an embodiment, the individualtubes have an aspect ratio of less than 20:1, less than 10:1, less than5:1 or less than 3:1. A useful dimension would be 0.5 mm by 3 mm. In anembodiment, the maximum dimension of the inlet is less than 5 mm and theminimum dimension is at least 3 mm. A tube which has a high aspect ratiowith the long side facing the center of the barrier structure 12 may bemost efficient because liquid will be removed all along its length. Rateof gas extraction is dependent on the cross-sectional area of the inletof the tube so that a round inlet will require a higher gas extractionrate for a given liquid extraction rate than a slit with a high aspectratio for the same rate of liquid extraction. In an embodiment, a lowgas extraction rate is desired. In one embodiment, the jet of the gasknife may be directed such that gas from the gas knife is directed toimpinge on the meniscus of the liquid rather than directly down towardsthe substrate.

In an embodiment, the extractor 420 is formed as a continuous slit. Theslit may be in the shape of an annulus. In order for this to work it maybe that the slit needs to be narrow to ensure that the radially inwardgas flow into the slit pushes the liquid radially inwardly. This ensuresthat the slit does not get locally clogged with liquid. For a gap heightbetween the barrier member 12 and the substrate W of about 100 μm, aslit width of less than 150 μm may achieve the above requirements.

FIG. 12 shows a second embodiment of the extractor in which the tubeshave individual separate inlets as opposed to being a groove which hasbeen split up into individual tubes as is the case in FIG. 11 a. As canbe seen from FIG. 12, a meniscus of liquid 800 forms between the tubes.The breakdown of the meniscus occurs when the velocity componentperpendicular to the meniscus surface exceeds a given value. Thus, itcan be seen that by elongating the shape of the gas knife such as inFIG. 10 in the scan direction (y) the component of the scan velocitywhich is in that direction perpendicular to the meniscus surface isdecreased thereby allowing a faster scan speed. In an embodiment, thediameter of the tubes is between 0.5 mm and 2.5 mm. However, in anembodiment, only one tube may be provided. In an embodiment, theseparation of the tubes is less than 10 times their maximum dimension.Thus, in the case of tubes with a circular outlet with a diameter of 1mm, the maximum separation of the tubes should be 10 mm. Of course evenin the embodiment of FIG. 12, the tubes could have an inlet shape, inplan, which is not circular but which is square and which could have asaspect ratio of up to 20:1 but, in an embodiment, less than 10:1, lessthan 5:1 or less than 3:1.

At the bottom of FIG. 12 there are illustrated some optional features indashed lines which may be used all together or individually incombination with the apparatus shown in solid lines. Under somecircumstances (particularly at high scan speed e.g. above about 700mm/second, or with a liquid with a low surface tension (e.g. highrefractive index liquids which often have low surface tension) at a scanspeed of above about 500 mm/second), the apparatus of FIG. 12 may notcontain all of the liquid. In this circumstance liquid tends to build-upin the bottom of the v notch of the gas knife 410 and then drip off theend of the v leaving a trail of liquid. Thus, in the embodiment of FIG.12 one option is provide a further extractor 470 at the tip of therecess of the first gas knife 410 on a side of the first gas knife 410opposite to the individual separate inlets. If the further extractor 470is attached to a low pressure source, liquid which escapes through thegas knife 410 can be collected.

Alternatively or in addition to the extractor 470, a further second gasknife 475 can be provided adjacent the tip of the first gas knife 410.The gas knife 475 may be in the form of a v such that liquid escapingfrom the region of the recess of the first gas knife 410 is directedtowards the center of the v of the second gas knife 475 where a furtherextractor 476 is positioned. Again the extractor 476 is connected to anunder pressure source. Only one of the two further extractors 470, 476may be present.

Alternatively, the second gas knife 475 can be provided as a straightgas knife as illustrated at the bottom of FIG. 12 and as labelled 480.In this instance further extractors 485 are provided at each end of thegas knife to extract any liquid which is caught by gas knife 480.

One difficulty with the embodiment of FIG. 12 is that the last bit ofliquid can be hard to extract, for example, because of the small volumeof the remaining liquid. One way to solve that problem is to maneuverthe extractor after drying of the surface of interest such that theplace where the last liquid collects (e.g., in the bottom of the v shapein the FIG. 12 embodiment) is positioned over an extractor or drain, forexample, in a top surface of the substrate table WT.

One difficulty with the embodiments of FIGS. 11 and 12 is that thesuction towards the substrate table WT generated by the under pressurein the liquid extractor 420 can deform the substrate table WT. One wayof dealing with this is illustrated in FIG. 13 in which a furtheropening 415 is provided between the gas knife 410 and the liquidextractor 420. The further opening 415 is open to ambient pressureP_(amb). This limits the under pressure generated between the barrierstructure 12 and the substrate table WT and also ensures that enough gasis available for the desired flow through the extractor 420. Typically agas knife has a gas flow of about 100 liters/minute. 50 percent of thatgas moves radially inwardly and 50 percent moves radially outwardly. Onthe other hand, the flow of gas desired through the liquid extractor 420can be as high as 70 liters/minute so that an additional 30liters/minute of gas is needed. In the embodiment of FIG. 13 thisadditional gas is provided by the further opening 415. The furtheropening 415 may be in the form of a slit (i.e. in the form of acontinuous groove) or may be a plurality of discrete holes.

In some instances the barrier member 12 is provided with a liquid outletsimilar to the outlets 50 in a bottom surface which, in use, faces thesubstrate W. Thus, a flow of liquid is provided down towards thesubstrate. Such a flow of liquid is useful in filling the gap between anedge of the substrate W and the substrate table WT. This flow of liquidis useful in reducing the inclusion of bubbles from the gap between thesubstrate W and substrate table WT when the barrier member 12 passesover the edge of the substrate W. This feature is described in moredetail in U.S. patent application publication no. US 2007-0081140, whichis incorporated herein in its entirety by reference. If such a supply ofliquid is used, this can help prevent damage in the event of loss ofcontrol of the apparatus. With a large under pressure being applied toliquid extractor 420 without measures being taken, a loss of control ofthe height actuator of the barrier member 12 could result in the barriermember 12 colliding into the substrate W or substrate table WT with alarge force. If the aforementioned liquid supply which supplies liquidin a direction towards the substrate W is provided, this can form aliquid bearing and protect the system from accidental loss of control(at least to some extent). Of course the aforementioned liquid supplycan also fulfill its main function of reducing bubble inclusion duringimaging of the edge of a substrate W.

One way of controlling the under pressure applied to the liquidextractor 420 is by controlling a suction pump to achieve a certain gasflow rate. Such control is particularly suitable during start up but hasa disadvantage that if the barrier member 12 for any reason moves closerto the substrate W or substrate table WT then the under pressuregenerated by the liquid extractor 420 increases. This is particularlyunfavorable in the event of loss of control of the spacing between thesubstrate W or substrate table WT and the barrier member 12.Furthermore, the performance of the actuator controlling the position ofthe barrier member 12 may suffer as a result of such intrinsicnon-linear behavior. One way of circumventing this problem is to connectthe liquid extractor 420 to an under pressure source of a fixedpressure. In that way the extraction pressure is substantiallyindependent of the size of the gap between the bottom of the barriermember 12 and the top surface of the substrate W so that machine safetyissues are minimized. In a hybrid system, a pump attached to the liquidextractor 420 can be controlled on start up to reach a certain desiredflow rate and for normal use can be controlled to achieve a certaindesired (perhaps constant) under pressure.

In a further embodiment, the liquid extractor 420 is not provided atdiscrete locations along the periphery of the space as illustrated inFIGS. 7-10. Instead tubes with outlets facing the substrate W such asthose illustrated in FIG. 11, 12 or 13 are provided (spaced evenly)around the entire periphery of the space filled with liquid. Theindividual tubes can be such as those illustrated in FIG. 11 (i.e.elongate in cross-section) or can be circular in cross-section asillustrated in FIG. 12. Radially outwardly of the tube outlets are aplurality of further openings 415 as described above in relation to FIG.13. Radially outwardly of those further openings 415 is provided a gasknife 410 also as described above.

FIG. 14 shows another embodiment of the present invention. In FIG. 14 asubstrate table WT is illustrated which supports the substrate W. Aliquid supply device LSD is provided and the substrate W and substratetable WT move under the stationary liquid supply device LSD andstationary projection system PS. Two gas knives 1100, 1120 are providedon either side of the liquid supply device LSD but are not attached tothe liquid supply device. The two gas knives 1100, 1120 are shaped asV's with the point of the V pointing towards the central axis 1200 ofthe substrate W. In an embodiment, the two sides of the V meet at anacute angle of less than 45°.

In the embodiment illustrated in FIG. 14 the gas knives 1100, 1120 arestationary in the y (scan) axis relative to the projection system PS butthey are arranged to step in the x direction in synchronization with thesubstrate table WT and substrate W. In an embodiment, the gas knives1100, 1120 are dynamically isolated from the liquid supply device LSDperhaps by being attached to a base frame of the lithographic apparatus.The base frame supports the substrate table WT whereas the referenceframe RF supports the projection system PS and is dynamically isolatedfrom the base frame BF.

Shaded area 1300 represents a film of liquid which is left behind by theliquid supply system as the substrate W moves under the liquid supplydevice (in the scan direction) and the projection system PS. In theillustration, the substrate table WT and substrate W are being scanneddownwards in the y direction. When the liquid hits the gas knife, as inthe other embodiments, the liquid is transported along the gas kniferadially outwardly. The substrate table WT is supplied with a drain 1400which surrounds the substrate W. Thus, liquid which is transported bythe combination of the gas knife 1100, 1120 and the movement of thesubstrate table WT reaches the drain 1400 and can thereby be removed.

It will be noted that the embodiment of FIG. 14 does not have a gasknife 1100, 1120 which surrounds the liquid supply device (though in oneembodiment it may have a gas knife). A gas knife which surrounds theliquid supply device is unnecessary as during the scanning of thesubstrate table WT and substrate W under the projection system PS thegas knives 1100, 1120 will act like “windscreen wipers” and clear anyliquid on the substrate. In the embodiment where the gas knives 1100,1120 step with the substrate WT in the step (x) direction, the gas knifeshould be arranged to extend along the whole diameter of the substrate.Furthermore the gas knife is angled relative to the x direction (i.e.does not lie in the x direction) such that the combination of thescanning movement and the gas knife lead to transport of the liquidtowards the drain 1400. Of course the gas knives 1100, 1120 do notnecessarily need to be V shaped or indeed straight but this is likely anefficient arrangement.

Desirably the distance between the liquid supply device LSD and the gasknives 1100, 1120 is large enough such that, in use, the film of liquidleft on the substrate behind the liquid supply breaks up into dropletsbefore the arrival of the gas knife 1100, 1120. Such droplets are easierfor the gas knives 1100, 1120 to deal with than a film.

In one or more of the above embodiments the film of liquid 600 leavingthe liquid supply system LSS has a free top surface which isunconstrained by a surface. This free surface enables the gas knife tobe decoupled from the meniscus.

While one or more embodiments have been described with respect to liquidremoval from a substrate, one or more embodiments may be applicable toother areas, e.g., the substrate table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination, of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The immersion liquid used in the apparatus may have differentcompositions, according to the desired properties and the wavelength ofexposure radiation used. For an exposure wavelength of 193 nm, ultrapure water or water-based compositions may be used and for this reasonthe immersion liquid is sometimes referred to as water and water-relatedterms such as hydrophilic, hydrophobic, humidity, etc. may be used.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus, comprising: a projection system configuredto transfer a pattern from a patterning device onto a substrate; aliquid supply system configured to provide liquid to a space between theprojection system and the substrate; a gas knife arranged to at leastpartly surround the space, the gas knife comprising a plurality ofsegments, each segment being elongate and having an inlet for gas; and aliquid removal device positioned adjacent to and along a fraction of thegas knife, the liquid removal device located outwardly, relative to anoptical axis of the projection system, of an imaginary circle centeredat the optical axis intersecting the segments.
 2. The apparatus of claim1, wherein the liquid removal device extends along less than 0.05 of thegas knife.
 3. The apparatus of claim 1, wherein the shape of the gasknife in plan is elongated in a scan direction.
 4. The apparatus ofclaim 1, wherein the gas knife is stationary with respect to the liquidsupply system.
 5. The apparatus of claim 1, wherein the liquid removaldevice is situated at a region at which neighboring segments join. 6.The apparatus of claim 1, comprising four segments and each segment issubstantially straight.
 7. The apparatus of claim 1, wherein the gasknife is a first gas knife and further comprising a second gas knifepositioned radially outwardly of the first gas knife with respect to theoptical axis.
 8. The apparatus of claim 7, wherein the second gas knifeis positioned to intersect a line which passes through the optical axisand the liquid removal device.
 9. The apparatus of claim 7, furthercomprising a further liquid removal device configured to remove liquidon which gas from the second gas knife impinges.
 10. The apparatus ofclaim 1, wherein the gas knife is shaped such that, on movement of thesubstrate in any direction in the plane of the substrate and whichpasses through an optical axis of the apparatus, in an arc of at least36°, liquid on the substrate in a plane which is perpendicular to theplane of the substrate and which contains the direction will, bycombined effect of the gas knife and further movement of the substratein the direction, be transported along the gas knife to the liquidremoval device.
 11. A lithographic apparatus, comprising: a projectionsystem configured to transfer a pattern from a patterning device onto asubstrate; a liquid supply system configured to provide liquid to aspace between the projection system and a substrate; and a gas knife atleast partly surrounding the space to contain liquid left on thesubstrate by the liquid supply system, wherein the gas knife comprises aplurality of curved elongate segments that do not all extend on a samecircle, each segment having an inlet for gas, wherein at least somesegments have a local radius relative to a center non-coincident with acenter of the gas knife of less than 0.9 of the average radius of thegas knife relative to the center of the gas knife or of greater than 1.1of the average radius of the gas knife relative to the center of the gasknife.
 12. A lithographic apparatus, comprising: a projection systemconfigured to transfer a pattern from a patterning device onto asubstrate; a liquid supply system configured to provide liquid to aspace between the projection system and a substrate; and a gas knife atleast partly surrounding the space to contain liquid left on thesubstrate by the liquid supply system, wherein the gas knife comprises aplurality of curved elongate segments that do not all extend on a samecircle, each segment having an inlet for gas, wherein a local radius ofa segment relative to a center non-coincident with a center of the gasknife is at least ten times larger than the average radius of the gasknife relative to the center of the gas knife.
 13. A lithographicapparatus, comprising: a projection system configured to transfer apattern from a patterning device onto a substrate; a liquid supplysystem configured to provide liquid to a space between the projectionsystem and a substrate; a gas knife at least partly surrounding thespace to contain liquid left on the substrate by the liquid supplysystem, wherein the gas knife comprises a plurality of curved elongatesegments that do not all extend on a same circle, each segment having aninlet for gas; and a liquid extractor, the extractor positioned outsideof an imaginary circle drawn at an average radius of the gas kniferelative to a center of the gas knife.
 14. A lithographic apparatus,comprising: a projection system configured to transfer a pattern from apatterning device onto a substrate; a barrier structure configured to atleast partly surround a space between the projection system and thesubstrate and to at least partly constrain liquid in the space, thebarrier structure comprising an outlet to provide liquid to the space;and a gas knife physically separate from the barrier structure, the gasknife sized, shaped and oriented such that, on movement of the substrateunder the barrier structure and under the gas knife, liquid left on thesubstrate following passage of the substrate under the barrier structureis transported along the gas knife to a liquid removal area.
 15. Theapparatus of claim 14, wherein the liquid removal area is a drain in asubstrate table configured to hold the substrate.
 16. The apparatus ofclaim 15, wherein the drain is configured to surround the substrate. 17.The apparatus of claim 14, wherein the gas knife is attached to theliquid supply system.
 18. The apparatus of claim 14, wherein the gasknife is moveable independently of the liquid supply system.
 19. Theapparatus of claim 18, wherein the gas knife is arranged substantiallyto move in unison with the substrate in a step direction.
 20. Theapparatus of claim 14, wherein the gas knife is supported by a baseframe of the lithographic apparatus from which the projection system, orthe liquid supply system, or both, are dynamically isolated.
 21. Theapparatus of claim 14, wherein the gas knife is arranged to be at anangle to a step direction of the apparatus.
 22. The apparatus of claim14, wherein the gas knife is arranged so as to extend along the wholewidth of the substrate in a step direction of the apparatus but not in ascan direction, wherein the scan and step directions are substantiallyorthogonal.
 23. The apparatus of claim 14, comprising two gas knives,each situated on an opposite side of the liquid supply system.
 24. Theapparatus of claim 14, wherein the gas knife is V shaped, with the pointof the V substantially aligned with a line of symmetry which splits thesubstrate in half in a scan direction of the apparatus.
 25. Theapparatus of claim 14, wherein part of the gas knife is situated oneither side of a liquid removal area.
 26. The apparatus of claim 25,wherein liquid can be transported along the parts of the gas knife oneither side of the liquid removal area towards the liquid removal area.27. The apparatus of claim 14, wherein the liquid left on the substratehas a free top surface.
 28. The apparatus of claim 14, wherein the gasknife is substantially straight or is made up of a plurality ofsubstantially straight portions.
 29. The apparatus of claim 28, whereina liquid extraction device is positioned at a junction between straightportions of the gas knife.
 30. A lithographic apparatus, comprising: aprojection system configured to transfer a pattern from a patterningdevice onto a substrate; a barrier structure configured to at leastpartly surround a space between the projection system and the substrateand to at least partly constrain liquid in the space; and a gas knifepositioned spaced apart from the barrier structure, wherein liquid onthe substrate which has escaped from the barrier structure has a freesurface between the barrier structure and the gas knife.
 31. Animmersion lithographic apparatus, comprising: a projection systemconfigured to transfer a pattern from a patterning device, through aliquid, onto a substrate; a liquid removal device configured to removeliquid from a surface of the substrate, the liquid removal devicecomprising a plurality of extraction tubes connected to a suctionsource, with an end, in use, directed towards the substrate andpositioned within ten times the maximum tube end plan dimension of eachother; and a further liquid removal device configured to remove liquidfrom the surface of the substrate that escapes the liquid removaldevice.
 32. The apparatus of claim 31, wherein the tube ends have, inplan, an aspect ratio of less than 20:1 mm.
 33. The apparatus of claim31, wherein the tube ends have, in plan, a maximum dimension of lessthan 5 mm.
 34. The apparatus of claim 31, wherein the tube ends arelined up in a line.
 35. The apparatus of claim 34, wherein the line issubstantially V shaped.
 36. The apparatus of claim 34, wherein the twosides of the V meet at an acute angle of less than 45°.
 37. Theapparatus of claim 31, further comprising a gas knife positionedadjacent the liquid removal device.
 38. The apparatus of claim 37,wherein the gas knife extends beyond the liquid removal device.
 39. Theapparatus of claim 31, further comprising an opening positioned radiallyoutwardly of the extraction tubes, the opening being in fluidcommunication with a gas source such that, in use, gas is drawn out ofthe opening and towards at least one of the plurality of extractiontubes.
 40. The apparatus of claim 31, wherein the ends of the pluralityof extraction tubes are positioned, in plan, around the entire peripheryof an area to which the liquid is provided.
 41. An immersionlithographic apparatus, comprising: a projection system configured totransfer a pattern from a patterning device, through a liquid, onto asubstrate; a first gas knife; a liquid removal device comprising aplurality of tubes connected to a suction source, ends of the tubespositioned adjacent the gas knife, the first gas knife and the pluralityof tubes extending in a V shape; and a second gas knife outwardly,relative to an optical axis of the projection system, of the first gasknife and the plurality of tubes, the second gas knife being shorter inlength than the first gas knife.
 42. The apparatus of claim 41, whereinthe tube end has an aspect ratio of less than 5:1.
 43. The apparatus ofclaim 41, wherein a maximum dimension of the tube end is 5 mm or less.44. The apparatus of claim 41, further comprising a liquid extractorpositioned to extract liquid from between the first and second gasknives.
 45. A device manufacturing method comprising projecting apatterned beam of radiation using a projection system through a liquid,supplied by a liquid supply system, onto a substrate and transportingliquid left on the substrate following passage of the substrate underthe liquid supply system along a gas knife to a liquid removal area, thegas knife comprising a plurality of segments, each segment beingelongate and having an inlet for gas and the liquid removal area locatedoutwardly, relative to an optical axis of the projection system, of animaginary circle centered at the optical axis intersecting the segments.46. A device manufacturing method comprising projecting a patterned beamof radiation using a projection system through a liquid onto asubstrate, removing liquid from a surface of the substrate using aplurality of extraction tubes which are connected to a suction source,have an end directed towards the substrate and positioned within tentimes of the maximum tube end dimension of each other, and removingliquid from the surface of the substrate that escapes the liquid removaldevice.